The present invention relates to systems for amplifying electronic signals. More particularly, and not by way of limitation, the present invention is directed to a system and method for canceling crosstalk between multiple channels using load impedance measurements.
Driving a stereo headset is a common requirement in today's mobile phones. There is a requirement to minimize the number of pins in the headset connector, and also to adhere to the standard headset connector found on most home music equipments. Typically, the standard headset has a three-terminal connector with left, right, and ground terminals. No DC current is allowed to flow through the headset. This requires the left and right signals to be an AC signal with a zero-volt DC offset. Such a signal may be generated using an amplifier with a positive and negative voltage supply. However, a negative supply is not readily available in a device operated by a single battery.
FIG. 1A is a simplified schematic drawing of a common configuration of stereo amplifiers for generating a stereo signal (i.e., left signal and right signal). The signal, Vin1 is fed into a first single-ended output amplifier (Output AMP1) 11, and the signal Vin2 is fed into a second single-ended output amplifier (Output AMP2) 12. The output amplifiers are providing the signal to a load such as headphones, speakers, etc. (not shown). The output amplifiers have a common-mode DC voltage equal to VDD/2. To prevent this voltage from creating a DC current flow through the load, DC-blocking capacitors (CL1 and CL2) 13 and 14 are used. The DC-blocking capacitors are needed in the absence of a negative voltage supply. A drawback with the DC-blocking capacitors is that they typically are 100-200 μF, each of which occupies significant area on a printed circuit board (PCB).
FIG. 1B is a simplified schematic drawing of another common configuration of stereo amplifiers for generating a stereo signal. This configuration utilizes a reference voltage supply (VMID) 15. The VMID driver is implemented as a reference amplifier (Reference AMP) 16 and provides half the voltage of the power supply (VDD/2) as a reference DC voltage level. A first output load (RL1) 17 is connected between Output AMP1 11 and the Reference AMP. A second output load (RL2) 18 is connected between Output AMP2 12 and the Reference AMP. The main reason for using the Reference AMP is to eliminate the DC blocking capacitors CL1 and CL2, thereby reducing the PCB area occupied and reducing the number of pins in the headphone jack.
FIG. 2 illustrates a problem that arises when using the Reference AMP 16 for the output amplifier loads. With this configuration, it is difficult to avoid crosstalk between the channels. The primary source of crosstalk is an output impedance (Rint) 19 in the Reference AMP 16. Crosstalk is injected from one channel to the other via this internal Reference AMP output impedance, Rint. If Rint is 1 ohm, and the load is 32 ohms, the crosstalk will be −30.1 dB (Crosstalk=20 log 1/32). Generally, a small Rint is more costly than a larger Rint. A method that will allow higher output impedance with the same crosstalk performance would thus save cost.
Instability can also be a problem with the Reference AMP configuration. Different configurations of the amplifier load result in differing capacitive and inductive loads. Too much capacitive load on the amplifier can easily make it unstable. It is known that the stability of an amplifier can be improved by adding a serial resistor between the Reference AMP output and the capacitive load. The drawback of adding more serial resistance to the output, however, is that it increases crosstalk between the channels.
It would be advantageous to have a system and method of crosstalk cancellation that overcomes the disadvantages of the prior art. The present invention provides such a system and method.